refactor!: rename vim.pretty_print => vim.print

Problem:
The function name `vim.pretty_print`:
1. is verbose, which partially defeats its purpose as sugar
2. does not draw from existing precedent or any sort of convention
   (except external projects like penlight or python?), which reduces
   discoverability, and degrades signaling about best practices.

Solution:
- Rename to `vim.print`.
- Change the behavior so that
  1. strings are printed without quotes
  2. each arg is printed on its own line
  3. tables are indented with 2 instead of 4 spaces
- Example:
  :lua ='a', 'b', 42, {a=3}
  a
  b
  42
  {
    a = 3
  }

Comparison of alternatives:
- `vim.print`:
  - pro: consistent with Lua's `print()`
  - pro: aligns with potential `nvim_print` API function which will
    replace nvim_echo, nvim_notify, etc.
  - con: behaves differently than Lua's `print()`, slightly misleading?
- `vim.echo`:
  - pro: `:echo` has similar "pretty print" behavior.
  - con: inconsistent with Lua idioms.
- `vim.p`:
  - pro: very short, fits with `vim.o`, etc.
  - con: not as discoverable as "echo"
  - con: less opportunity for `local p = vim.p` because of potential shadowing.

test/functional/lua/commands_spec.lua

@@ -8,6 +8,8 @@ local eval = helpers.eval
 local feed = helpers.feed
 local clear = helpers.clear
 local meths = helpers.meths
+local exec_lua = helpers.exec_lua
+local exec_capture = helpers.exec_capture
 local funcs = helpers.funcs
 local source = helpers.source
 local dedent = helpers.dedent
@@ -15,7 +17,6 @@ local command = helpers.command
 local exc_exec = helpers.exc_exec
 local pcall_err = helpers.pcall_err
 local write_file = helpers.write_file
-local exec_capture = helpers.exec_capture
 local curbufmeths = helpers.curbufmeths
 local remove_trace = helpers.remove_trace
 
@@ -142,22 +143,29 @@ describe(':lua command', function()
     ]]}
   end)
 
-  it('Can print results of =expr', function()
-    helpers.exec_lua("x = 5")
-    eq("5", helpers.exec_capture(':lua =x'))
-    helpers.exec_lua("function x() return 'hello' end")
-    eq([["hello"]], helpers.exec_capture(':lua = x()'))
-    helpers.exec_lua("x = {a = 1, b = 2}")
-    eq("{\n  a = 1,\n  b = 2\n}", helpers.exec_capture(':lua  =x'))
-    helpers.exec_lua([[function x(success)
+  it('prints result of =expr', function()
+    exec_lua("x = 5")
+    eq("5", exec_capture(':lua =x'))
+    exec_lua("function x() return 'hello' end")
+    eq('hello', exec_capture(':lua = x()'))
+    exec_lua("x = {a = 1, b = 2}")
+    eq("{\n  a = 1,\n  b = 2\n}", exec_capture(':lua  =x'))
+    exec_lua([[function x(success)
       if success then
         return true, "Return value"
       else
         return false, nil, "Error message"
       end
     end]])
-    eq([[true    "Return value"]], helpers.exec_capture(':lua  =x(true)'))
-    eq([[false    nil    "Error message"]], helpers.exec_capture(':lua  =x(false)'))
+    eq(dedent[[
+      true
+      Return value]],
+    exec_capture(':lua  =x(true)'))
+    eq(dedent[[
+      false
+      nil
+      Error message]],
+    exec_capture(':lua  =x(false)'))
   end)
 end)